It came down the fall 2014 runway on the chest of a sporty black lamé sheathdress, on the thigh of some baggy blue jeans embroidered with a samurai emblem, and on the back of a kimono-shape fatigue jacket: revolution. The word was scrawled on a rectangular band of heat-transfer decals, and it served as a punchy slogan—along with uprising, twisted, and bunny hop—for the new order at Marc by Marc Jacobs. Leading that charge are the brand's longtime accessories consultant, Katie Hillier, now creative director of the women's collections, and her old friend and collaborator Luella Bartley, of the fondly remembered former label Luella, as ready-to-wear design director. Although Jacobs is still the boss, he appears to have given his two appointees—both British, blond, and punk in spirit—near-total carte blanche. From the get-go, they have proved theirs will not be a quiet riot. And while there has been no formal name change, the abbreviation seen on kneesocks in their debut show has caught on in the industry: MBMJ has an undeniable millennial ring to it.

Marc by Marc Jacobs was ready for a refresh. The younger, more affordable sibling to the megabrand had gone from pioneering graffiti-laden utilitarian chic to parading retro neutrality via an oft-revisited grunge theme. "It started out very bold, and then it began to look a bit cute and vintagey," Hillier says diplomatically of the label launched in 2000 by Jacobs and his business partner, Robert Duffy. Even Duffy has said that the brand had grown stale ("Our customer was wanting a little bit of the old magic and surprise back"), in part because he and Jacobs were so focused on Louis Vuitton, where Jacobs was creative director until last year. They are currently shoring up Marc Jacobs International for an initial public offering, which puts the secondary line in the primary position—it accounts for 70 percent of the company's net sales.

It may be a high-stakes gambit to cede control at such a critical moment, but Hillier, 40, and Bartley, 41, are surprisingly laid-back, exuding self-assurance and a palpably giddy excitement about their new gig. The day we meet at their studio in Shoreditch, London, it is the first of July, which just happens to be their first anniversary at the brand. Hillier: "Woo-hoo!" Bartley: "Yay! Let's celebrate!" The duo oversees a small RTW design team here—"mom-and-pop," as Hillier describes it—while the main operations, from fittings to fabric sourcing, take place in New York City. The night before, Bartley's partner, the fashion photographer David Sims, had a gallery opening, so she and Hillier are admittedly partied out. Bartley has a turmeric-laced detox juice; Hillier opts for coffee and Marlboro Lights.

Decadelong fans of Marc by Marc Jacobs, they describe their mission as modernization rather than radicalization of the brand. "I remember when we'd do the Luella shows in New York, the first place we'd go was the Marc by Marc store on Bleecker Street," Bartley reminisces. "At some point I suppose I stopped looking, because the marketplace was cluttered with imitations of what Marc had started."

Just who is the MBMJ girl? Judging from the designers' cartoony-cool first collection, she is one part ninja, one part (dirt) biker chick, and one part defiant romantic. The mash-up of looks for fall—tomboyish racing suits in a manga-motocross style, gingham taffeta blouses that look like Christmas presents, plastic pencil skirts, metallic tulle dresses, obi belts—telegraphs fearlessness spiked with charm. "That whole girl-power thing and a woman feeling confident in what she's wearing, whether she's 16 or 50, is really important to me and Katie," Bartley says, underscoring that MBMJ is not just for Generation Hashtag. "I've worn nothing but these clothes since the show."

Hillier and Bartley aren't the sort of wunderkinder frequently plucked to reinvent major houses these days, but their combination of Anglo cheek and industry cred makes them bull's-eye mascots for Marc by Marc Jacobs. A graduate of Central Saint Martins and a powerful force in fashion (the Gisele bag she designed for Mulberry in 2002 is credited with reviving that brand), Bartley ran a namesake label that specialized in contrarian girlishness and enjoyed a cult following for a decade before it folded in 2009—less than a year after she won Designer of the Year at the British Fashion Awards—due to financial difficulties. After shuttering her business, Bartley took a long hiatus in Cornwall, rode horses, and wrote a book, Luella's Guide to English Style.

She and Hillier met in 1999 through stylist Katie Grand, an editor and a key member of Jacobs' coterie of British collaborators; Bartley promptly hired University of Westminster–trained Hillier to handle accessories at Luella. Hillier also helped Victoria Beckham launch her handbag line and started Hillier London jewelry in 2010, with bijous featuring bunnies and rainbow stars—a fun-loving attitude she's translating for MBMJ in the form of striped, high-gloss mini satchels and chain-festooned bandannas. Meanwhile, the Luella influence shows up in voluminous satin skirts, capelets with massive bows at the neckline, and plaids galore.

At the moment, the codesigners' greatest concern is for consistency across the brand itself and between runway and retail. Essential to that is collating all the references that define the MBMJ girl's world, a process they do best at Bartley's kitchen table in Bloomsbury, listening to everything from Malcolm McLaren to Beyoncé, flipping through photography by the likes of Joseph Szabo and Nan Goldin, and scouring YouTube. Proving their pop savvy, they enlisted skateboard artist and graphic designer Fergus Purcell to devise new logo graphics for fall; for resort, there were patchwork button-downs with spacey prints by illustrator Zoë Taylor, who is also working on Bartley's second book, a fairy-tale collection. There's an all-in-the-family vibe to their approach and an unrelenting POV. A new Instagram campaign starring real people hired from a pool of submissions labeled #CastMeMarc and shot by Bartley's partner speaks to the personal, inclusive atmosphere the designers are aiming for. "We want to stand for something," Hillier says matter-of-factly. "I think that's why people love Marc by Marc Jacobs so much, because it's one of the last really accessible fashion collections where you can actually buy into that."